Biochemical Foundations of Human Vital Activity - Volkov N.I., Nesen E.N. 2000

Biochemical Foundations of Human Vital Activity
Biochemistry of Lipids
Characteristics of Lipid Classes

Depending on their molecular Structure, the fats found in The Human Body are divided into the following main classes: neutral fats, Phospholipids, Glycolipids, and Steroids (sterols and sterides).

A characteristic structural component of most Lipids is Fatty acids, which store the majority of The energy released during their oxidation. They appear in the body in free form following the Enzymatic Hydrolysis of triglycerides or their Biosynthesis in the adipose tissue of the Liver.

Fatty Acids

Fatty acids are organic acids with a long hydrocarbon chain (radical R)—containing from 4 to 24 or more carbon atoms—and a single carboxyl group. The general formula for fatty acids is С_nН2_n+1-СOOН, or R-COOH.

Many fatty acids are characterized by an even number of carbon atoms, which is presumably due to their synthesis by adding two-carbon units to the growing hydrocarbon chain.

The fats of the human body most commonly contain fatty acids with 16 or 18 carbon atoms, which are referred to as Higher Fatty Acids. Higher fatty acids are divided into saturated and unsaturated. The principal ones are presented in Table 16.

Class="center">TABLE 16. Higher Fatty Acids

In saturated fatty acids, all available carbon bonds are filled with hydrogen. Such fatty acids contain no double or triple bonds in their carbon chain. Unsaturated fatty acids contain double bonds in their carbon chain , the first of which occurs between the ninth and tenth carbon atoms counting from the carboxyl group.

Fatty acids with triple bonds are rare. Fatty acids containing two or more double bonds are called polyunsaturated. Stearic acid serves as an example of a saturated fatty acid, while oleic acid is an example of a monounsaturated fatty acid (having one double bond), as can be seen from their Spatial Models and structural formulas:

Among the higher saturated fatty acids, palmitic (С₁₆) and stearic (С₁₈) are the most common in the human body, whereas oleic (С₁₈), linoleic (С₁₈), linolenic (С₁₈), and arachidonic (С₂₀) are the most prevalent unsaturated ones.

As the number of carbon atoms in fatty acid molecules increases, their melting point rises. Fatty acids can be solid substances (e.g., stearic acid) or liquid (e.g., linoleic and arachidonic acids); they are insoluble in Water and very sparingly soluble in alcohol.

Unsaturated fatty acids are more reactive than saturated ones. They readily add two hydrogen or halogen atoms (iodine, chlorine) at the sites of their double bonds, thereby converting into saturated ones:

This process is known as hydrogenation. Substances that undergo hydrogenation change their properties. For instance, vegetable oils are converted into solid fat. The hydrogenation reaction is widely used to produce solid dietary fat—margarine—from liquid vegetable oils.

Polyunsaturated fatty acids are of particular importance to humans. They cannot be synthesized within the body. If they are absent from the diet, Lipid METABOLISM—particularly Cholesterol Metabolism—is disrupted, and pathological changes occur in the liver, Skin, and platelet function. Therefore, unsaturated fatty acids such as linolenic and linoleic are essential nutritional factors. Furthermore, they facilitate the export of synthesized fats from the liver, thereby preventing hepatic steatosis. This action of unsaturated fatty acids is called the lipotropic effect. Unsaturated fatty acids serve as precursors for the synthesis of BIOLOGICALLY ACTIVE SUBSTANCES known as Prostaglandins. The normal daily requirement for polyunsaturated acids in humans is approximately 15 g.

Neutral Fats

Neutral fats comprise a group of lipids consisting of the trihydric alcohol glycerol and three fatty acid residues, which is why they are called triglycerides, such as tripalmitin:

Neutral fats may contain identical fatty acids, such as palmitic acid. In this case, a simple fat, or triglyceride known as tripalmitin, is formed. If the fats contain different fatty acids, mixed fats are formed, the name of which depends on their constituent acids. For example, the triglyceride 1-palmitoyl-2-oleoyl-3-stearin contains palmitic, oleic, and stearic acids:

Natural fats feature A wide variety of constituent fatty acids, their various arrangements within the molecule, and varying degrees of unsaturation. Potentially, millions of triglyceride isomers can exist.

The PHYSICOCHEMICAL PROPERTIES OF fats are largely determined by their fatty acid composition. Fats containing predominantly saturated fatty acids are solid at room Temperature, whereas those rich in unsaturated fatty acids are liquid. Solid fats are generally of animal origin, with the exception of fish oil. Liquid fats are vegetable oils, except for coconut and palm oils, which solidify upon cooling. In both animal and plant organisms, unsaturated fatty acids are twice as abundant as saturated ones.

Neutral fats accumulate in fat Cells (adipocytes) beneath the skin, in the Mammary Glands, and in the adipose capsules surrounding the internal abdominal Organs; small amounts are also found in skeletal Muscles. The formation and accumulation of neutral fats in adipose tissues is termed deposition. Triglycerides form The basis of reserve fats, which serve as the body's energy storage and are utilized during fasting, inadequate fat intake, or prolonged physical exertion.

Neutral fats are also constituents of Cell membranes and complex protoplasmic Proteins, in which case they are referred to as protoplasmic fats. Protoplasmic fats are not utilized as an energy source even during severe emaciation, as they serve a structural function. Their amount and chemical composition are constant and independent of dietary intake, whereas the composition of reserve fats is constantly changing. In humans, protoplasmic fats account for about 25% of total body fat mass (2–3 kg).

ENZYMATIC REACTIONS OF neutral fat BIOSYNTHESIS AND DEGRADATION constantly take place in various Cells of the body, particularly in adipose tissue:

The hydrolysis of fats in the body yields glycerol and free fatty acids. This process is catalyzed by Enzymes known as lipases. The breakdown of fats in body tissues is called lipolysis. The rate of lipolysis increases significantly during endurance exercise, and lipase activity becomes enhanced with physical training.

When fat degradation is carried out in the presence of alkalis (NaOH, KOH), sodium or potassium salts of fatty acids are formed, which are called soaps, and the reaction itself is known as saponification:

This chemical reaction forms the basis of soap manufacturing from various fats and their mixtures.

Phospholipids

Phospholipids are fat-like substances composed of an alcohol (most commonly glycerol), two fatty acid residues, a phosphoric acid residue, and a nitrogenous substance (an alcohol such as Choline, an amino acid such as Serine, etc.). Currently, about 25 different subclasses of phospholipids, differing in molecular composition, are distinguished. The general structural scheme of phospholipids is as follows:

Phospholipids are widely distributed in various body tissues. Of particular importance are choline phosphatides, colamine phosphatides, and serine phosphatides, which are derivatives of phosphatidic acid and contain various nitrogenous bases:    

Choline phosphatides, or lecithin, are found in large quantities in egg yolks. In the human body, they are widely distributed in Nervous Tissue.

Phospholipids play a vital biological role as Structural components of all cell membranes and as suppliers of choline, which is necessary for the Synthesis of the neurotransmitter acetylcholine. Membrane properties such as permeability, receptor function, and the catalytic activity of membrane-bound enzymes depend on phospholipids.

A distinct group of membrane phospholipids is represented by Sphingolipids, which contain the unsaturated amino alcohol sphingosine instead of the alcohol glycerol. The most common of these phospholipids are sphingomyelins. They are involved in The formation of the myelin sheaths of Nerve Cells.

Glycolipids

Glycolipids may contain various alcohols, such as glycerol or sphingosine, along with CARBOHYDRATES and other substances; however, they lack phosphoric acid. Glycosphingolipids, specifically cerebrosides and gangliosides, are of great importance in the human body.

Cerebrosides contain the alcohol sphingosine, fatty acids, and various sugar residues, namely D-galactose or D-glucose. Cerebrosides containing D-galactose (galactocerebrosides) are found predominantly in the White matter OF the Brain, whereas cerebrosides containing D-glucose (glucocerebrosides) are present in the membranes of other cells.

Gangliosides are structurally the most complex sphingolipids. They contain several sugar residues as well as residues of N-acetylneuraminic (sialic) acid. Gangliosides are found in the grey matter of the brain, where they make up about 6% of Membrane Lipids. They are detected in smaller amounts in The Cell membranes of other tissues. In addition, gangliosides serve as components of specific receptor sites located On the surface of cell membranes—that is, at the sites where neurotransmitter molecules bind during the Chemical Transmission of nerve impulses from one neuron to another.

Steroids

Steroids are fat-like substances characterized by the presence of the complex tetracyclic ring system of sterane (cyclopentanoperhydrophenanthrene).

Important natural steroids include Bile acids, male and Female Sex Hormones, Adrenal hormones, and certain toxins. These steroids are usually present in cells in small amounts. They perform essential biological Functions in the body: they integrate into the Structure of Cell membranes and regulate specific physiological functions. Steroids in the body are represented by sterols and sterides.

Sterols are high-molecular-weight cyclic alcohols containing a hydroxyl group at the C-3 position and a hydrocarbon side chain at the C-17 position of the sterane ring. Cells are very rich in sterols. The most prevalent sterol in body cells is cholesterol:

Cholesterol was first isolated from gallstones (from the Greek chole, meaning bile) in the 17th century. It is a crystalline substance that is insoluble in water. In the body, it plays a vital role as a precursor in the synthesis of bile acids, Steroid Hormones, and vitamin D₃. Cholesterol increases the resistance of red Blood Cells to hemolysis and activates The Citric Acid Cycle. It is hypothesized that in the brain, cholesterol acts as a specific insulator protecting brain structures from electrical charges during the propagation of nerve impulses.

Sterides are esters formed by sterols and higher fatty acids. The fatty acids that make up sterides are primarily palmitic, stearic, and oleic acids. However, the sterides of lanolin (a waxy substance found in animal skin and wool) have been found to contain myristic, arachidonic, cerotic, and other branched-chain complex fatty acids.

All sterides are solid, colorless substances (from the Greek steros, meaning solid). In the animal body, they typically occur as complexes with proteins.